This invention relates to a corona generating element and more particularly to a corona generating element for use in electrophotography.
Corona generating elements are used in electrophotography to apply a uniform electrostatic charge to the surface of a photoconductive member. The charge applied may be either positive or negative depending on the nature of the photoconductive member. If the charge applied to the surface of the member is not uniform, the electrostatic image formed on the surface on exposure to a light pattern will not correspond exactly to the light pattern but will result partly from variations in the unformity of the applied charge.
In U.S. Pat. No. 4,025,339 to M. R. Kuehnle, there is described a photoconductive member of extremely high sensitivity that is capable of reproducing or creating high resolution images, that is, each point on the surface of the member is capable of selectively discharging in accordance with the intensity of incident light so that an almost infinite scale of grey tones can be reproduced on the resultant image. The photoconductive member accepts a charge of negative polarity. In order to make full use of the exceptional features of this photoconductive member, it is necessary that the charge applied be substantially uniform over its entire surface.
One type of corona generating element that is commonly used in electrophotography is in the form of a strand of fine metal wire of circular cross-section. The diameter of the wire is usually in the order of about 75 microns. The wire is stretched taut between anchoring end pieces and connected to a source of high voltage of sufficient magnitude to ionize the air about the strand. The ions so produced are then attracted to the photoconductive member by any suitable means. Some of the advantages of this type of corona generating element are that it is simple in construction, is easy to fabricate, inexpensive to fabricate, and will produce a substantially uniform corona throughout its length when used in the positive mode. One of the disadvantages of this type of corona generating element is that it will not produce a uniform corona when it is used in the negative mode. More specifically, it has been found that when a strand of metal wire is used to generate a negative corona, the corona so produced will have a tendency to vary in density from point to point along its length. It is believed that this nonuniformity is caused by randomly located and shifting hot spots which develop for one reason or another along the wire and become sources of intense radiation. In order to compensate for these hot spots and to produce a corona along the wire that is substantially uniform, various approaches have been suggested and/or actually put into use. For example, in U.S. Pat. No. 3,958,162 to M. R. Kuehnle, a plurality of metal wire corona generating elements are positioned parallel to and rotated about a central axis and in U.S. Pat. No. 3,978,380 to P. G. Talmage, a single metal wire corona generating element is reciprocated about its longitudinal axis. Examples of other approaches that have been suggested and/or actually put into use to compensate for hot spots in a metal wire corona generating element may be found in U.S. Pat. No. 2,856,533 to J. F. Rosenthal and U.S. Pat. No. 3,233,156 to J. G. Jarres et al.
Another type of corona generating element that is frequently used in electrophotography is in the form of an array of needle- or pin-shaped electrodes conductively connected to each other. When a high voltage source is connected to the electrodes, a corona is generated around the tipe of each electrode. Although this type of corona generating element does not have the problem of randomly located hot spots, it does have other types of problems. One of the main problems is that the electrodes must be spaced a relatively large distance apart to produce a relatively uniform corona at voltages around 6 KV and if one electrode should fail to operate there is a sizeable area over which no corona is generated. Examples of this type of corona generating element may be found in U.S. Pat. No. 3,581,149 to H. Tanaka et al.; U.S. Pat. No. 3,624,392 to A. Kurahashi, etc.; U.S. Pat. No. 3,649,380 to M. Sato et al.; U.S. Pat. No. 3,691,373 to R. G. Compton, etc.; U.S. Pat. No. 3,765,154 to Hardt et al.; and U.S. Pat. No. 3,959,690 to K. Leibrecht.
Examples of other types of corona generating elements used in applications other than electrophotography may be found in U.S. Pat. No. 2,852,093 to R. G. Streuder, U.S. Pat. No. 3,294,971 to J. C. Von De Hade, and U.S. Pat. No. 3,766,382 to H. Lin.